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Cassini Significant Events 12/09/15 – 12/15/15

Cassini is currently orbiting Saturn with a period of 12.7 days in a plane inclined 1.3 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Dec. 16 using the 70-meter diameter Deep Space Network station in Australia. The spacecraft continues to be in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at http://saturn.jpl.nasa.gov/news/significantevents/anomalies .

Commands from Cassini's S92 sequence controlled basically all of the spacecraft's activities at Saturn this week. Back on the home planet, Sequence Implementation Process teams continued working on additional 10-week command sequences. S93 will begin executing on Feb. 7, and S94 starts on April 18. Milestones have also been slated for S95 activities. As usual, developing command sequences includes negotiating with other users of the Deep Space Network (DSN), to schedule antennas for Cassini weeks or months in advance, so that the spacecraft will be able to operate in sync with its counterpart of ground tracking facilities.

Cassini’s International Scientist for a Day essay contest for students in grades 5-12 is now in full swing. Students are invited to choose one of three observing opportunities and provide rationale for their choice. Check out http://saturn.jpl.nasa.gov/education/scientistforaday2015/ for full details.

Wednesday, Dec. 9 (DOY 343)

Cassini's Ultraviolet Imaging Spectrograph (UVIS) began controlling the spacecraft's attitude for 38 hours today, to carry out a mosaic scan across Saturn’s equatorial region. This is the first in a series totaling 62 hours of observations this week. The scans consist of three swaths, one with the instrument's slit centered on the equator, one with it pointed north of the equator, and one to the south.

Thursday, Dec. 10 (DOY 344)

Five hundred days from now, Cassini will make its final targeted flyby of Saturn's huge moon Titan; the encounter is known as T-126. The gravity-assist nudge Cassini will take from Titan's gravitation and orbital momentum during the flyby will fling the spacecraft into its final series of 22 highly inclined orbits, the Proximal Orbits. Each of their apoapses will occur 1.27 million kilometers "above" Saturn, just beyond the orbit of Titan. The periapses, though, will be quite remarkable: positioned in between Saturn's atmosphere and the innermost of its rings (the D ring), they will provide extraordinary opportunities for close-in science observations. As it passes periapsis, the spacecraft will be going over 120,000 km per hour relative to the planet, nearly 20 times its velocity at apoapsis.

Friday, Dec. 11 (DOY 345)

The Imaging Science Subsystem (ISS) took the reins for 30 minutes to make an observation in the satellite orbit campaign, looking to recover small objects near the planet or discover new ones. Next, UVIS started a nine-hour scan, the second one of its Saturn-mosaic swaths. The Visible and Infrared Mapping Spectrometer (VIMS) made its own observations while riding along.

Saturday, Dec. 12 (DOY 346)

ISS, VIMS and the Composite Infrared Spectrometer (CIRS) performed a nine-hour observation edge-on to the E ring while it was sunlit at a moderate phase-angle. This is the wide, diffuse ring of Saturn that Enceladus creates by virtue of its icy geysers. Following this, UVIS took over pointing control for 15 hours to make its final observation in this series of Saturn-mosaic scans. ISS and VIMS rode along.

Sunday, Dec. 13 (DOY 347)

While UVIS was making its scans, Cassini coasted through apoapsis, the high point in its 12.7-day orbit of Saturn. Upon reaching a height of 1.95 million km from Saturn's center, the spacecraft had slowed to 6,058 km per hour relative to the planet. This marked the start of Saturn Orbit #228.

Monday, Dec. 14 (DOY 348)

This week, Cassini scientists gathered to participate in the Fall 2015 meeting of the American Geophysical Union (AGU) in San Francisco.

The month-long Radio Science superior conjunction experiment, which used Cassini's continuous radio signals to study the solar corona, came to an end today. The spacecraft is now more than 13 degrees west of Sun in our sky.

Saturn's irregular moon Skathi (also written Skadi) was named after a giantess in Norse mythology. Only about six km in diameter, this very dark-surfaced little object follows an orbit that reaches as far as 1.97 million km from the planet, in an inclined retrograde path. ISS began a 21-hour observation of this object today, tracking it in the narrow-angle camera's telescope.

A remarkable image of Saturn's moon Enceladus, taken while it was transiting in front of Tethys, which is twice the width of Enceladus, was featured today. Enceladus's smoother, snowy surface is clearly evident /resources/16284 .

Tuesday, Dec. 15 (DOY 349)

Cassini's Navigation team had an opportunity today to execute an Orbit Trim Maneuver (OTM), which would fire the rocket thrusters to adjust the spacecraft's approach to Saturn's moon Enceladus. Orbit-determination solutions, using the most recent radiometric tracking data from the DSN (line-of-sight Doppler-shift and range data) showed that Cassini was so close to its intended path that the maneuver, OTM-433, would not be necessary. Therefore, the flight team's tasks to design, build, approve, and uplink commands to execute an OTM were cancelled. Instead, the flight team updated the onboard ephemeris with pointing tweaks for both the Enceladus and Aegaeon encounters this weekend. The Enceladus encounter, E-22 on Dec. 19, will be Cassini's final targeted encounter with that extremely interesting moon.

On four occasions this week, while Cassini was pointing its optical instruments towards or nearly towards Saturn, ISS turned and made two-minute Saturn storm-watch observations. VIMS rode along with three of them. Also during the week, the Deep Space Network communicated with and tracked Cassini on five occasions, using stations in Australia and California. A total of 15 individual commands were uplinked, and about 1,300 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.

This illustration shows Cassini's position early on Dec. 15: http://go.nasa.gov/1QSHDJg . The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects (except the background stars of course) revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.